1. Field of the Invention
The present invention pertains generally to apparatus for controlling the voltage applied to one or more high intensity discharge lights for purposes of conserving power and more specifically to an improved transformer apparatus for advantageously lowering the voltage applied to a lighting system after initial activation thereof.
2. Prior Art
The present invention is particularly suited to the function of permitting the application of a normal high voltage to a lighting load, such as a bank of high intensity discharge street lights and to thereafter selectively reduce the voltage to maintain such lights in a lighted condition but with reduced power consumption. The general concept of voltage reduction for the aforementioned purpose is well known in the art. By way of example, U.S. Pat. No. 2,429,162 issued Oct. 14, 1947 to Kaiser et al, discloses a number of alternative transformer configurations, utilizing a variety of switches and relays. These configurations permit the application of a nominal voltage to a plurality of fluorescent lights, followed by a reduction in that voltage subsequent to lamp lighting in order to maintain the lights in a lighting condition at a reduced power consumption. U.S. Pat. No. 4,189,664, issued Feb. 19, 1980 to Hirschfield, discloses another type of transformer configuration, utilizing a switch for selectively applying one of a plurality of taps from an autotransformer to a lighting load to reduce the voltage delivered to the load to thereby reduce the power consumed by the load.
The prior art concept which utilizes an autotransformer and a switching means for either selectively applying one or more taps of the transformer to the load or shorting portions of the autotransformer for controlling the voltage applied to the load, serves the aforementioned purpose of reducing power consumption in a light or a lighting load, but unfortunately suffers from a number of disadvantages which the present invention overcomes. By way of example, in such prior art disclosures, the switch unit that is used to control the voltage applied to the load is normally interposed between the line power and the load. As a result, it must be capable of supporting the entire load current on either the input or output power of the autotransformer. To withstand such high current carrying requirements, such switching devices are large and bulky and are commensurately expensive. Furthermore, their large size makes it difficult to integrate such power consumption reducing devices into existing street lighting because the large package size cannot be easily accommodated in the existing lighting structure and must therefore be specially installed underground, requiring expensive and time consuming installation procedures.
Other relevant patents that disclose the use of relay controlled transformer windings for load power consumption reduction include the following. U.S. Pat. No. 2,180,193 to Brand, U.S. Pat. No. 2,853,654 to Swasey and U.S. Pat. No. 3,652,824 to Okada. Perhaps the most relevant prior patent is U.S. Pat. No. 4,431,948 issued to Elder et al, on Feb. 14, 1984. The parent application of which the present application constitutes a continuation-in-part, discloses an improvement over that prior patent that was made necessary to solve a significant disadvantage. This disadvantage occurred during maximum voltage application when the relay or equivalent switching device was in its open configuration, thus disconnecting the common winding of the autotransformer from the series winding of the autotransformer. It was founded that in the full voltage mode, the current in the series winding produced a significant magnetic field which induced magnetic flux saturation in the metal portion of the common winding which caused two problems. One such problem was that the magnetic field caused the common winding to produce a voltage which caused the common winding, even though disconnected electrically from the series winding to induce harmonic distortion in the series winding which effectively reduced the amplitude of the voltage available from the series winding during the full voltage configuration. The reduction in voltage was significant and in some cases making it difficult or impossible to turn on fluorescent or other arc type light systems during the high voltage start-up mode. Another problem associated with the magnetic flux saturation of the common winding metal structure was that it heated the metal structure of the autotransformer so significantly that it tended to destroy the insulation in the coil which could result in catastrophic effects. The aforementioned parent application discloses a solution to that disadvantage, namely the inclusion of a bucking coil in parallel with the common winding of the autotransformer which is switched into that configuration only when the common winding is disconnected from the series winding. The autotransformer is thus provided with an additional winding, namely a bucking coil winding which is wound in bifilar opposite relation with the common winding of the autotransformer. This bifilar bucking coil winding is, because of its opposite direction of winding, designed to cancel out all of the ill effects described with respect to the unitary common winding of the prior art. Consequently, despite the continuing high magnetic field generated by the series winding of the autotransformer in the high voltage mode, any harmonics generated in the common winding are, in effect, cancelled by the equal and opposite generation of harmonics in the bucking coil. In fact, any voltage generated in the common winding as a result of magnetic effects on the common winding by the current flow through the series winding, are exactly cancelled by the equal and opposite effects generated in the bucking coil winding. As a result, no heat is generated of any significance in the metal structure of the autotransformer and harmonic distortion of the full voltage alternating sine wave is eliminated to the point where the full voltage or virtually full voltage can be achieved in that mode, thus enabling the necessary high voltage to initiate the arc start of high intensity discharge lights, such as street lights disclosed herein.